2.1. Polymer Networks
Synthetic polymer networks have been the subject of extensive theoretical, physical, and chemical study over the past century (i,ii), and are still finding new applications (iii,iv). The structurally simplest polymer networks are termed “model networks” (MNs) and are typically comprised of linear telechelic, or α,ω-functional, polymers, or macromonomers (MAC), covalently crosslinked through their end groups with multi-functional small molecules. Macromonomers are defined as oligomers with a number average molecular weight Mn between about 1,000 and about 10,000 that contain at least one functional group suitable for further polymerizations. MNs are unique because the crosslink functionality is constant and predetermined, so that the molecular weight between crosslinks is defined by that of the MAC, and the material is homogenous with respect to the crosslink density. (v). Well defined-pore sizes are therefore obtained, providing potential advantages for certain applications. (vi). Although MNs have well-defined structure, they are not considered ‘ideal’ in a theoretical sense, because they unavoidably contain some number of unreacted functionalities, dangling chains, chain entanglements, and inelastic loops. (v).
Due to their insolubility in all solvents, MNs are notoriously difficult to characterize by common chemical techniques. As a consequence, certain network parameters, like the number of dangling chains, are typically estimated from combining macroscopic measurements (swelling, rheology, etc. . . . ) with theory. Recent research has utilized a hydrolytically labile crosslinker for the degradation of cross-linked star-polymer model networks (CSPMNs), and size exclusion chromatography (SEC) of the degradation products to verify the parent network structure. (ill). However, such CSPMNs have been prepared successfully only through the use of methyl methacrylate (MMA) monomers. If applied to networks of linear MACs, analysis of degradation products can also, in principle, yield the number of dangling chains after subtracting out the sol portion.
Recently, the development of a controlled/“living” free radical polymerization technique known as Atom Transfer Radical Polymerization (ATRP), described in Wang, J-S, and Matyjaszewski, K., Journal of the American Chemical Society, Vol. 117 (1995), p. 5641, has rendered possible the synthesis of a variety of well-defined polymers with low polydispersity indexes (Mw/Mn<1.3, where Mw is the weight average molecular weight) and predetermined molecular weights, defined by the relationship DP=Δ[M]/[I]0, where DP is the degree of polymerization, [M] is the reacted monomer concentration, and [I]0 is the initial concentration of the initiator. The mechanism of ATRP, shown in Scheme 1 below, is believed to be based on the repetitive addition of a monomer M to growing radicals R. generated from alkyl halides R—X by a reversible redox process. This process is catalyzed by transition metal compounds, especially cuprous (Cu(I)) halides, complexed by suitable ligands such as bipyridines and bi-, tri- and tetradentate amines, as described in Xia, J. Zhang, X. and Matyjaszewski, K., American Chemical Society Symposium Series, Vol. 760 (2000), pp. 207-23. The rate of monomer addition is dependent on the equilibrium constant between the activated (Cu(I)) and deactivated (Cu(II)) species. By maintaining a low concentration of active radicals, slow growth of the molecular weight is promoted and the “living” ATRP process is controlled. The degree of polymerization is determined by the ratio of reacted monomer concentration to initiator concentration (DPn=Δ[M]/[R−X]0).

Radical reactions allow for polymerization of a large variety of vinyl monomers and are tolerant to many functional groups. ATRP is applicable to the reactions of hydrophobic monomers such as acrylates, methacrylates and styrene, as shown in Patten, T. E. and Matyjaszewski, K. Advanced Materials, Vol. 10 (1998), pp. 901-915, and also of hydrophobic and functional monomers such as 2-hydroxyethyl acrylate, 2-hydroxyethyl methacylate, 2-(dimethylamino)ethyl methacrylate (DMAEMA) and 4-vinylpyridine. See Matyjaszewski, K., Gaynor, S. G., Qiu, J., Beers, K., Coca, S., Davis, K., Muhlebach, A., Xia, J., and Zhang, X., American Chemical Society Symposium Series, Vol. 765 (2000) pp. 52-71.
Further, researchers have recently reported the copper (I) catalyzed azide-alkyne cycloaddition (CuAAC) reaction (vii, viii), which has emerged as the best example of “click chemistry,” (ix) characterized by extraordinary reliability and functional group tolerance. This ligation process has proven useful for the synthesis of model polymers and materials in many situations.
2.2. Degradable Polymers
A degradable polymer is a polymer that contains a cleavage site, a bond in the chemical structure that will cleave under certain conditions. Degradable polymers have many applications, such as drug delivery, medical devices, environmentally-friendly plastics, and temporary adhesives or coatings. A variety of natural and synthetic polymers are degradable. Generally, a polymer based on a C—C backbone tends to be non-degradable, while heteroatom-containing polymer backbones are degradable. Degradability can therefore be engineered into polymers by the addition of chemical linkages such as anhydride, ester, or amide bonds, among others.
Biodegradable polymers with hydrolyzable chemical bonds have been the subject of extensive research. Polymers based on polylactide (PLA), polyglycolide (PGA), polycaprolactone (PCL) and their copolymers have been extensively employed as biodegradable materials. Degradation of these materials yields the corresponding hydroxyacids, making them safe for in vivo use.
Photodegradable polymers can be created by the addition of photosensitive groups (promoters) to the polymer. Two common promoters are carbonyl groups and metal complexes, which cleave when exposed to sufficient ultraviolet radiation, such as that present in sunlight. However, metals left behind by cleavage of these heavy metal complexes can cause environmental problems in sufficient quantities.
Degradable model networks in particular have many potential applications, yet in order to be successfully used, a method of yielding MACs of low polydispersity that possess orthogonal crosslinking and various degradation functionalities is necessary.